It is often necessary to perform maintenance on portions of vessels which are underwater. As an example, when a vessel is in dry dock the keel rests on keel blocks, which remain in place until the vessel is refloated. The areas covered by the keel blocks must then be cleaned, painted, or otherwise treated to prevent the spread of corrosion. There are many other tasks, such as welding, carried out on vessels and other submerged structures, which require the provision of an air or other gas filled work chamber around the area being serviced. As used herein, the term "gas" includes air and other gases.
Many different types of air chambers, habitats, cofferdams and the like have been devised. Most of these must be clamped, tied, or otherwise secured to the work area, which can be difficult to do on a large vessel. Some devices are very complex and expensive for the simple purpose that they serve.